Host-guest complexes have been used in the field of perfumery and for malodour counteracting for some time. For example, U.S. Pat. No. 5,942,217 describes the use of an aqueous solution of cyclodextrin, more generally referred to as cyclic oligosaccharides, for neutralizing malodour. Absorption of the malodour molecule in the cyclodextrin cavity results in efficient mitigation of malodour. The malodour binding process occurs during drying, and low molecular weight polyols can be added to the composition to enhance the formation of cyclodextrin inclusion complexes, especially in the case where the malodour molecule is too small to form a stable inclusion complex. Fragrance molecules may be added to the aqueous solution, and less than about 10% of the cyclodextrin complexes with these molecules.
U.S. Pat. No. 7,208,464 describes cosmetic compositions, comprising between 2 and 20% water, more than 50% of a volatile solvent, such as ethanol, a fragrance oil and a material, such as cyclic oligosaccharides, which is able to provide prolonged, and noticeable, “top note” characteristics. It is believed that when a composition is applied to a substrate an association exists between the perfume raw materials and the entrapment material such that the evaporation of the perfume raw materials is delayed. Over time, this association breaks down resulting in release of the perfume raw materials. The result is that the “top note” character continues to be experienced by the user over time.
However, in U.S. Pat. No. 7,919,452 C1-C8 alkyl-substituted cyclic oligosaccharides having an average degree of substitution from 1.6 to about 2.8 are described as delaying the release of volatile solvents and also reducing the initial harsh alcoholic odour impact of an alcoholic or hydro-alcoholic cosmetic composition. It is described that the volatile solvent itself competes with the perfume oils for docking in the cyclic oligosaccharide cavity. This results in some “in situ complex formation” between the volatile solvent and the cyclic oligosaccharide. It is believed that the stability profile of this cyclic oligosaccharide—volatile solvent complex is such that there is a delay in the release of the volatile solvent from the composition when applied to a substrate. It is further believed that this results in a perceptible reduction in initial solvent release thus satisfying the consumer desire for a reduced initial harsh solvent odour upon application of the composition.
As is apparent from the above prior art, the action of cyclodextrin on the perception of both malodours and perfumes depends strongly on the activity of water in the system. This reflects complex equilibria involving water, fragrance and malodour inclusion in the cyclodextrin cavity. Hence, the apparent host-guest binding constant may vary for both fragrances and malodours, depending on the concentration of water in the system. The poor predictability of such, essentially non-equilibrium effects, is a source of recurring difficulties for the perfumer. The poor selectivity of cyclodextrin binding and release with respect to fragrances and malodours is a disadvantage. Although this effect can be beneficial in the context of the controlled release of a desirable odour, it becomes very unsatisfactory if the guest molecule released is a malodour.
Another disadvantage of cyclodextrins is the tendency of these carbohydrate-derived materials to become sticky at low water levels or with decreasing water activity. This can lead to, for example, an undesirable “tacky” feeling on the skin. Finally, as is usually the case with aqueous carbohydrate solutions, aqueous cyclodextrin solutions must be preserved against micro-organism invasion using significant amounts of preservatives.
In view of the disadvantages associated with cyclodextrin, alternative host-guest systems are desirable which overcome these problems. In U.S. Pat. No. 6,869,466, a method is described to bind a gas or a volatile molecule to a cucurbituril to form an inclusion complex as well as steps for releasing at least some of the bound gas or volatile compounds. Trapping malodorous compounds is mentioned as an example of application. The formation of the host-guest complex is described as occurring in both dry state, for example with the cucurbituril host molecules adsorbed or supported on a solid surface or dissolved or dispersed in a liquid, for example water.
WO2014077641 describes a composition comprising a complex of cucurbit[7]uril and a fragrance molecule for use in odour removal and provides some evidence that a fragrance molecule is released when an odour molecule is bonded to the cucurbit[7]uril host molecule.
In a pro-fragrance, the fragrance is generally bound covalently to an odourless substrate or another fragrance. The pro-fragrance may be synthesized by a chemist or may occur spontaneously in a mixture. Known examples of spontaneous pro-fragrances are Schiff bases that occur following the reversible reaction between anthranilates and aldehydes. When applied to a surface, such Schiff bases have a low to moderate odour, while after a certain time and under the action of moisture and temperature, both powerful anthranilate and aldehyde are released, providing a prolonged perception of both molecules on the surface. Pro-fragrances therefore have a variety of applications in fragrance related products.
Although cucurbituril host-guest complexes overcome many of the disadvantages associated with cyclodextrin complexes, there is a need for a pro-fragrance composition where the fragrance performance attributes, including perfume accord and release profile, can be selected and optimised according to objective criteria.